PWM (pulse-width-modulation) electric energy conversion devices are electrical devices adapted to transfer electrical energy from an input source, generally a DC voltage source, to an output, generally with AC voltage. Inverters of this type are for example used to convert DC voltage electrical energy supplied by a renewable energy source into AC electrical energy to be fed into a distribution grid, for example a 50 Hz or 60 Hz grid. In other applications, the inverter can be used to supply a load, for example an electric motor, at AC voltage.
In general, inverters used to convert energy coming from a renewable source, such as a photovoltaic panel or the like, can supply AC voltage electrical energy to be fed alternately into a distribution grid or to supply a local load, according to need and as a function of the quantity of electrical energy available from the DC source.
The electrical energy source can for example be a photovoltaic panel or a field of photovoltaic panels, a wind turbine generator or a group of wind turbine generators, or even a source with fuel cells or the like.
In many applications it is necessary to ground the DC voltage energy source. For example, in the case of photovoltaic panels, the positive or negative terminal of the panel or of the series of photovoltaic panels, is usually grounded to prevent degradation of the panel caused by the accumulation of charges on stray capacitance to ground. In other situations, for example in the case of supplying power to electric motors, the motor casing is grounded to prevent phenomena of erosion caused by leakage currents.
Grounding of a single machine or of a single inverter is normally a simple operation, but some problems may occur in the case of a plurality of devices which share the same AC voltage output. In this case each inverter has a ground connection and the outputs are connected in parallel with one another.
If, for example, the inverters are connected to a distribution grid, the neutral of which has a different potential to the potential of the ground connection point of the inverters, strong leakage currents are generated, which flow through the electronic switches of the inverters with the risk of destroying them.
Inverters are normally connected in parallel on a LV/MV (Low Voltage/Medium Voltage) transformer. To avoid the recirculation of high ground currents on several inverters, it is necessary to use multiple windings output transformers on the low voltage side, so as to obtain galvanic isolation between the inverters and therefore avoid the recirculation of high intensity currents through the ground connection. A configuration of this type is very onerous.
The problem of leakage currents in the case of a system with several electrical energy sources connected to a plurality of inverters connected in parallel to one another to an electrical distribution grid will be better illustrated with reference to FIGS. 1 and 2. However, it must be understood that similar problems can occur not only in the case of connection to the distribution grid, but also, for example, when two or more parallel inverters supply a common load.
FIGS. 1 and 2 refer in particular to a system, in which two photovoltaic inverters are each connected to a respective photovoltaic panel or to a field of photovoltaic panels, while the outputs of the two inverters are connected in parallel to a three-phase electrical distribution grid. PV1 and PV2 indicate the two fields of photovoltaic panels connected respectively to a first inverter 1 and to a second inverter 2. In the example illustrated, each inverter 1, 2 is a two-stage inverter and comprises a DC/DC stage and a DC/AC stage. The three-phase outputs of the two inverters 1, 2 are connected in parallel to a three-phase grid GR, the neutral of which is indicated with N.
On the input side each inverter has a plurality of bulk capacitors in series. In the case illustrated, four bulk capacitors are provided, indicated with C1/1, C2/1, C3/1 and C4/1 for the inverter 1 and with C1/2, C2/2, C3/2 and C4/2 for the inverter 2. The DC voltage electrical energy source is connected across the series arrangement of the two central capacitors. More precisely, the positive pole of the source PV1 is connected between the capacitor C1/1 and the capacitor C2/1, while the negative pole of the source PV1 is connected between the capacitor C3/1 and the capacitor C4/1. Moreover, the positive pole of the source PV2 is connected between the capacitor C1/2 and the capacitor C2/2, while the negative pole of the source PV2 is connected between the capacitor C3/2 and the capacitor C4/2. T1 and T2 indicate the central points of the series of bulk capacitors of the two inverters 1 and 2. Each inverter is therefore an inverter with four voltage levels. Each DC/DC stage charges the capacitors C1/1 and C4/1 of the inverter 1 and the capacitors C1/2 and C4/2 of the inverter 2. For the inverter 1 the capacitor C1/1 and the capacitor C4/1 are charged using energy drawn from the capacitor C2/1 and C3/1, respectively. Likewise, for the inverter 2 the capacitor C1/2 and the capacitor C4/2 are charged using energy drawn from the capacitor C2/2 and C3/2, respectively.
It shall be noted that, in general PV1 and PV2 can be different sections of a same field of photovoltaic panels, or different fields of photovoltaic panels also separated spatially. The dimension of the two fields of photovoltaic panels and/or the conditions of solar irradiation or other parameters can be different for the sources PV2 and PV2, so that the voltages V1 and V2 at the output of the two electrical energy sources will in general differ from one another.
For the aforesaid reasons, each inverter is grounded. In the example illustrated the ground connection is made in the point E1 for the source PV1 and the related inverter 1 and in the point E2 for the source PV2 and the related inverter 2. In the example illustrated, the ground connection points are on the negative terminal, but this is only an example, it being understood that the ground connection could also be made on the positive terminal. The considerations set forth below are also valid in this second case.
As the two voltages V1 and V2 are generally not identical, the voltages VC3/1 and VC3/2 across the capacitor C3/1 for the inverter 1 and across the capacitor C3/2 for the inverter 2 will normally also be different and equivalent to ½(V1) and ½(V2) respectively. Due to the ground connection, if the aforesaid two voltages are different, in the absence of appropriate measures there will be a leakage grounding current, as can be understood from the equivalent circuit represented in FIG. 2. Ileak indicates the leakage current. As a result of the connection in parallel of the AC output of the two inverters, then the voltage between the neutral N of the three-phase networks Gr and the central point T1 (isolated ground point) of the series of bulk capacitors of the inverter 1 is equal to the voltage between the neutral N and the central point T2 (ground) of the series of bulk capacitors of the inverter 2. In other words, the following relation is valid:VNT1=VNT2  (1)
This condition, which is represented here for a system with two inverters, is valid for all the inverters connected to the same network transformer, for example a same LV/MV transformer.
The current which can flow through the ground connection is virtually unlimited, i.e. limited only by the stray resistance and can cause damage to the inverters as well as constituting a serious risk for the correct operation of the system.
As mentioned above, to avoid the occurrence of a leakage current to ground Ileak in the state of the art a LV/MV transformer is normally used, provided with a plurality of low voltage windings galvanically isolated from one another, each of which is connected to one of the inverters of the system. This implies very high costs.